In preliminary studies we have demonstrated that chick hyaline cartilage contains in addition to Type II collagen, small amounts of three collagen chains originally call 1a, 2a and 3a (Burgeson and Hollister, 1979). We have also isolated two new disulfied-bonded collagenous molecules from cartilage which we call the higher molecular weight (HMW) and low molecular weight (LMW) fractions. Together, HMW and LMW represent about 5-8% of the pepsin-soluble collagen of chick cartilage. Amino acid compositions of 1a, 2a, HMW and LMW show similarities to Type V collagen, but cartilage does not appear to contain any of the a1(V) and a2(V) chains found in other tissues. In future experiments, a detailed structural analysis of HMW and LMW will be performed including the preparation and characterization of CNBr peptides, and the electron microscopic examination both after rotary shadowing and the precipitation of segment long spacing crystallites. The biosynthesis of 1a, 2a, HMW and LMW will be investigated in chick chondrocyte cultures and the procollagens of these molecules will be isolated after DEAE-cellulose chromatography. controls on the synthesis of 1a, 2a, HMW and LMW will be investigated in conditions where collagen "switching" occurs from Type II collagen to Type I collagen and Type I trimer. A specific antibody will be prepared against the native form of HMW and used in immunofluorescence staining at the light microscopic level both to localize HMW in cartilage and to determine the time of initiation of HMW biosynthesis during cartilage differentiation. The antibody will also be used to immunoprecipitate the procollagen form of HMW from the medium of chondrocyte cultures. at present very little information exists regarding the structure of HMW, LMW, 1a and 2a, and nothing is known of the function of these molecules. Chick sternal cartilage will serve as a useful model to study these collagens, the tissue being readily available and the chondrocytes growing easily in tissue culture. When more structural information is available describing the new cartilage collagens, it will be possible to decide their function, and to determine if they are involved in the degenerative diseases of articular cartilage.